Cellulose pulp can be made of wood either by mechanical processing, by refining or grinding, or chemically by cooking. The pulps produced by these methods contain lignin, which causes the brown color of pulp and has to be removed in order to produce white paper.
Most of the lignin in cellulose pulp prepared by cooking degrades during cooking. However, a part of it remains in the pulp and must be removed before directing the pulp to further processing. Removal of lignin takes place in the washing and bleaching stages following pulp cooking. In the bleaching stage bleaching chemicals are added to the pulp, which chemicals cause the brightness of the pulp to increase.
Peroxides are effective bleaching chemicals and they have been used bleaching mechanical pulp for a long time due to their high bleaching effect and environmentality. Namely, no environmentally harmful substances, for example organic chlorine compounds, are created in peroxide bleaching as they are created in bleaching methods based on chloride. The bleaching effect of the peroxide is based on oxidizing the colored chromophore radicals of lignings and rendering them colorless. Recently peroxide bleaching has been used more and more also in chemical pulp bleaching. An advantage of peroxide bleaching is the increased fiber gain of cooking, because the kappa number of the pulp to be bleached needs not be zero, but the kappa number may vary from 2 to 6 and a good bleaching result is still reached. As a result of this, the fiber gain from cooking increases. Generally peroxide bleaching is the last stage in the bleaching plant of chemical pulp.
The bleaching effect of peroxides is based on the formation of perhydroxyl ion OOH− in an alkaline solution. The following reaction shows the dissociation of hydrogen peroxide into a OOH− ion:
            H      2        ⁢          O      2        ⁢            →      ←        pH    ⁢            O      ⁢                          ⁢      O      ⁢                          ⁢              H        -              +          H      +      
The reaction is an equilibrium reaction, which depends on the temperature and pH of the solution.
FIG. 1 shows the dependence of the reaction between perhydroxyl ion and hydrogen peroxide on the pH and temperature of the solution. As can be seen from the diagram, the importance of temperature on the formation of perhydroxyl ion is small. However, the pH of the solution has a conclusive significance in the formation of perhydroxyl ion. It can be seen in the diagram that when the pH is below 9, hydrogen peroxide is approximately 100% in peroxide molecular form. Whereas when the pH is over 11, i.e. very alkaline, approximately 100% of hydrogen peroxide is in perhydroxyl ion form.
Thus, in peroxide bleaching the aim is to maintain the pH of pulp suspension on such a range that the perhydroxyl ion content would be high enough and that the desired final brightness target of pulp would be reached. The pH is controlled by means of an alkaline chemical, generally sodium hydroxide (NaOH), but magnesium oxide (MgO) can also be used.
Currently the peroxide bleaching stage is controlled by measuring the brightness of the bleached pulp and the amount of peroxide and pH in the suspension both before and after the peroxide bleaching tower. The brightness of the pulp is compared with the desired target brightness and on the basis of that and the peroxide measurements and pH the amount of peroxide and sodium hydroxide supplied to the bleaching tower is controlled. The measurements are generally performed as laboratory measurements. Currently the measurements can also be performed with on-line and/or in-line analyzers.
The level of the in-line or on-line measurements measuring the amount of peroxide and especially the condition of the measuring devices performing the measurements are tested by measuring samples from the pulp suspension at certain intervals in a laboratory. By comparing the laboratory measurements and the measurements of the on-line measuring devices it is possible, for example, to conclude whether the measuring device is in working order. The laboratory measurements as such are not well suited for controlling the bleaching stage due to their slowness. The most common used laboratory measurements are based on determining the amount of total peroxide, which is performed by titrating.
For measuring the amount of peroxide in the suspension as on-line measurement, measuring methods based on electrochemical measuring are generally used. These methods measure the electric current produced by a filtrate sample taken from the pulp suspension. The pulp suspension and thus also the filtrate sample contain both peroxide molecules and perhydroxyl ions. The electrochemical diffusion current created by a perhydroxyl ion is approximately ten times stronger than the electrochemical diffusion current created by the peroxide molecule. This means that this measuring method in practice measures the electric current created by the perhydroxyl ions of the sample and the perhydroxyl ion content, and is therefore not comparable with the laboratory measurement measuring the total peroxide content. Thus, the mutual correlation of these measurement is poor.
U.S. Pat. No. 6,332,951 discloses a method for measuring peroxide content of pulp suspension in the bleaching stage, which is based on catalytical decomposition of peroxide. In this method catalyst is added to a sample taken from the suspension, and the catalyst causes the peroxide to decompose into water and oxygen. The oxygen formed in the reaction vessel forces a part of the sample into a measurement chamber. The pressure of the measurement chamber corresponds to the total peroxide amount in the sample. The correlation of this measurement with the laboratory measurements is quite good, but the problem with; the measurement is its complexity as well as the deviations in the measurement results caused by sample handling and maintenance of the equipment.
U.S. Pat. No. 6,774,992 shows a method for determining the peroxide and/or perhydroxyl ion contents of a pulp suspension to be bleached. In the method of the publication the sample is analyzed optically by Raman spectroscopy and the peroxide and perhydroxyl ion contents are determined from the peak intensities of the measured spectrum. According to the publication these contents can, together with the brightness measurements performed on the pulp, be used in controlling the amount of peroxide solution used in bleaching. A problem with the measurement is cleansing of the optics in the measurement equipment and problems with the repeatability of measurements.
FI-publication 89517 (corresponding U.S. Pat. No. 4,878,998) shows a control method of a two-stage mechanical pulp bleaching process. In the method the brightness of the pulp is measured after the bleaching tower and the residual peroxide amount in the backwater received at the press from the pulp after bleaching tower. In addition, the pH of the backwater is measured. The measuring results are used in controlling the amount of peroxide supplied to the bleaching stage. A problem with the solution disclosed in this publication is that by measuring the total peroxide amount of the backwater no knowledge of how much of the peroxide is in an active perhydroxyl ion form is formed. As a result of this the amount of peroxide added to the bleaching tower is not correct.